Proper control of serum phosphate concentrations is required to maintain skeletal integrity. We previously identified missense mutations in Fibroblast growth factor-23 (FGF23) as the cause of autosomal dominant hypophosphatemic rickets (ADHR), characterized by hypophosphatemia secondary to isolated renal phosphate wasting and metabolic bone disease. We subsequently demonstrated that inactivating FGF23 mutations result in the mirror-image disorder to ADHR, familial tumoral calcinosis (TC), which is manifested by markedly elevated serum phosphate and often severe ectopic and vascular calcifications. Further, FGF23 is elevated in X-linked hypophosphatemic rickets (XLH) and increased circulating FGF23 is also associated with a 5-6 fold higher mortality risk in patients with chronic kidney disease (CKD). There are currently no cures, only maintenance treatments, for the aforementioned syndromes. Although much progress has been made towards understanding both basic and clinical aspects of phosphate metabolism, the fundamental mechanisms regulating Fgf23 at the level of the skeleton under normal conditions and in disease are unknown. FGF23 is expressed in osteoblasts and osteocytes, however the cell type(s) that 'sense'changes in serum phosphate to control production and secretion of this hormone are also unknown. Importantly, we recently determined that primary cultures of osteoblasts/osteocytes increase Fgf23 in response to PTH and to phosphate, which represents an innovative shift in the current models explaining Fgf23 regulation. In combination with the newly- proposed in vivo systems, the cultures provide a novel approach for testing cell-specific control of Fgf23. The available Fgf23 knock-out and transgenic mouse models have provided essential in vivo data regarding FGF23 bioactivity, including interactions with its co-receptor a-Klotho (KL), however there are several limitations, including that the Fgf23 global knock-out is severely compromised at weaning and dies at 8-10 weeks, and the transgenic mice uncontrollably over-express FGF23. Therefore, an Fgf23 conditional-null model is a necessary tool to explore the pathophysiologic effects of Fgf23 in a non-lethal state and for isolating its role in human disorders. These proposed studies will address the central hypothesis that: FGF23 is regulated in a cell-specific manner, which is altered in disease, through undertaking the following specific aims: 1) To develop a mouse that will carry an Fgf23 allele that can be specifically interrupted in the presence of Cre recombinase;and 2) To test for cell-specific regulation of Fgf23 utilizing the conditional-null Fgf23flox/flox mice. Successful development of an Fgf23 conditional-null animal will provide a unique and important resource for us and for other investigators. Most of the molecular mechanisms guiding Fgf23 production and regulation under normal circumstances and in disorders of mineral metabolism are unknown, therefore this model will permit testing of hypotheses that are otherwise not possible. PUBLIC HEALTH RELEVANCE: The regulation of serum phosphate concentrations is critical for normal skeletal formation and cellular function. Pathogenic disturbances in phosphate homeostasis involving Fibroblast growth factor-23 (FGF23), such as those in the Mendelian disorders autosomal dominant hypophosphatemic rickets (ADHR), X-linked hypophosphatemic rickets (XLH), autosomal recessive hypophosphatemic rickets (ARHR), and familial tumoral calcinosis (TC), or common disorders such as renal failure, lead to severe endocrine and skeletal disease. These disorders currently have inadequate treatments. We expect that development of an Fgf23 conditional-null animal will reveal new mechanisms involved in phosphate homeostasis, and will provide novel therapeutic targets.